Conventional electroded discharge lamps have three distinct discharge regions: the anode region; the cathode region; and the positive column between the two electrode regions. Radiation from the positive column accounts for most of the light produced by most discharge lamps. In contrast, the electrode regions generate light, if any, with significantly lower efficacy than that of the positive column. Therefore, the overall efficacy of a discharge lamp can be increased by increasing the percentage of total power delivered to the positive column, while decreasing the percentage of total power delivered to the electrode regions. The power delivered to the positive column is the product of the lamp current and the voltage drop across the positive column. Similarly, the power delivered to each of the two electrode regions is the product of the lamp current and the voltage drop between the positive column and each electrode. Therefore, since the same lamp current flows through all three discharge regions, the goal of reducing power loss in the electrode regions becomes the goal of reducing the voltage drop in the electrode regions relative to the voltage drop in the positive column.
It is well-known that the magnitude of the anode fall is dependent on the size of the anode. The reason is that the negative space charge about the anode, which accounts for the voltage drop, decreases as anode surface area increases. Electrons, therefore, require less energy to overcome the repulsive force of the space charge at the anode. Hence, the magnitude of the anode fall decreases as anode size increases. However, since a single electrode serves as an anode and as a cathode on alternate half cycles of the operating alternating current, cathode design considerations have heretofore prevented the mere enlargement of the electrodes of the discharge lamp. In fact, electrode design has heretofore been optimized for operation as a cathode due to its critical role as an electron emitter, while anode operation has been deemed the secondary consideration. More specifically, a cathode of large surface area is undesirable because the cathode must heat to thermionic electron emitting temperatures rapidly at the start of lamp operation in order to avoid destructive sputtering. Therefore, with cathode design as the foremost consideration, the electrodes are not large enough to collect electrons during the anode cycle which are moving at their thermal velocity in the discharge plasma, thus necessitating the inducement of an accelerating field between the plasma and the anode, or inducement of the anode fall.
In a standard fluorescent lamp, for example, electrons are emitted from the portion of the tungsten electrode which has been coated with a low work function electron-emitting substance well-known in the art, such as an alkaline earth oxide. When the same electrode operates as an anode, electrons are collected on the portion of the tungsten electrode which has not been coated with the electron-emitting substance and also on the uncoated electrode support wires. The power consumed when electrons are collected is equal to the product of the anode fall and the lamp current. This power heats the portion of the electrode where electrons are collected. Most of this power is wasted because the collecting portion of the electrode differs structurally from the emitting portion so as to emit light with relatively low efficiency. Further, in high current lamps, the power dissipated during the anode cycle can heat parts of the electrode structure to undesirable high temperatures. Additional wire anodes are, therefore, welded to the electrode in some high current fluorescent lamps to increase anode surface area, thereby reducing the temperature of the anode. These wire anodes, however, do not avoid the anode fall.
The electrode structure in high intensity discharge lamps differs from that in fluorescent lamps, but the basic operation is similar. That is, a single electrode serves as both the anode and the cathode; the design, therefore, is optimized for cathode operation.
Enlarged anodes or shield means have been employed in some discharge lamps to reduce the anode fall. These structures generally comprise an additional grid-like or shield-like member mounted in proximity with the lamp electrode which serves both the cathode and anode functions. However, these lamps are not widely used for several reasons. Foremost is the problem of the enlarged anode, or additional structure, acting as a cathode during cathode operation. For instance, upon starting the lamp, the anode may act as a cold cathode until the cathode becomes hot enough to emit electrons in the thermionic mode. This initial cold cathode operation causes sputtering from the anode and, thus, darkening of the lamp walls. Further, emission material which evaporates or sputters from the cathode deposits on the anode, thus making cathode operation of the anode with resultant sputtering more likely. The effect of sputtering is a reduction in light output of the lamp. Although the anode fall of these lamps may be reduced, it is not avoided. The overall energy saving of these lamps is minimal, if any.